Adding onboarding doc with excalidraw images

[maernest04]

PR for #150

Problem

For SCEvents, we have the original development team but we need to add more contributors to help maintain the project as time goes on. Currently, there is no onboarding documentation for SCEvents, making it difficult to add more people.

Solution

Added an onboarding document covering:

• Project architecture and dependencies
• Local setup and testing
• Domain concepts and registration flow
• API and Clark integration
• Debugging and recovery guidance
• Development and deployment workflows

[steeevin88]

nice addition + definitely helps with ramping up onto SCEvents

only potential gripe I have is updating this doc once the project changes/ new features are added; how can we prevent this some getting stale?

ex. we initially avoided super verbose code pointers in the READ.ME b/c we (rightly) anticipated code changes

ideally updating this doc is just automated but idk what budget would be needed, what do you think